Cyclone Debbie has pummelled Queensland state since crashing ashore as a category four storm on Tuesday between Bowen and Airlie Beach, ripping up trees, washing boats onto land and causing widespread damage.

It has been downgraded to a tropical low as it tracks southeast, but continues to pack damaging gusts and dump huge amounts of rain, with Brisbane now in the firing line.

Meteorologists said the city was soaked by a month's worth of rain in a single day, with the popular Sunshine Coast and Gold Coast areas also drenched ahead of the system moving offshore, expected Friday.

Theme parks and beaches in the area were closed.

"We have a very, very large state here and this is a very, very big weather system that's going to wreak havoc all the way down the coast," Queensland Premier Annastacia Palaszczuk said.

"Boats washed ashore, houses without roofs, windows smashed in, trees snapped in half, gum trees torn out of the ground and those that do remain standing, are bare and lifeless," she told the local Whitsunday Times.

The military has mobilised 1,300 soldiers to help assess the full extent of damage and aid the clean-up, with helicopters and planes deploying to restore infrastructure and supply emergency food, water and fuel.

Debbie has officially been declared a catastrophe by the Insurance Council of Australia, allowing claims from the disaster to be prioritised.

The economic cost to a region that relies heavily on tourism and farming is expected to be huge, with sugarcane crops hit hard and the cattle industry also impacted, officials said.

"Producers in the Whitsunday area were planting winter vegetables and they're expected to suffer heavy crop losses as well as infrastructure, crop and irrigation equipment damage," said Queensland's Rural Economic Development Minister Bill Byrne.

"In the Mackay district, it's understood that the cane fields at Proserpine, Mackay and Sarina have been flattened."